This invention lies in the field of flare stacks, for the purpose of emergency venting to the atmosphere of large quantities of combustible (waste of vent) gases. More particularly, it concerns the design of the flare stack, with means for the prevention of the inflow of atmospheric air into the top of the stack, and down along the inner surface of the stack, where it can mix with the outflowing combustible gases, and cause explosions.
In refineries, petrochemical plants and similar plants, there must be provided field flares which are designed for the emergency venting to the atmosphere of huge quantities of combustible gases, at irregular intervals. These are standby devices, which must be ready at all times to take the vented gases and burn them, as vented to the atmosphere. In between the times when the large flow of combustible gas is vented, while there is a minimum flow of purge gas, there is possibility of inflow of atmospheric air into the stack, and its flow down the stack, creating a combustible mixture in the stack which may, under certain conditions, explode and cause considerably damage.
While the emergency vented gases are flowing through the system, they are generally moving at high speed, of the order of 200 feet/second, or more, and turbulent flow conditions prevail. However, in the standby condition, when the vented gases are no longer flowing a standby flow of purge, or sweep, gases is provided. In view of the cost of the gas, its flow rate is minimal, of the order of 0.05 feet/second, more or less. The flow condition of the purge or sweep gases is laminar, or non-turbulent, at least for a very great preponderance of the time of flow. The laminar flow condition is that which makes avoidance of air entry to the flare stack and flare system difficult, because the prevention of air entry into the stack is due to the kinetic energy of the gases. In laminar flow, the velocity with which purge gases move is not uniform across the entire cross-section of the flare stack, and flare system tubular members, where pipe or round ducts are typically used.
Laminar flow is described in Mark's Mechanical Engineers Handbook, McGraw-Hill as follows:
"If the forced movement of fluid through a filled pipe occurs as a telescopic sliding of adjacent layers of fluid without transverse mixing, the resistance of this type of movement is due entirely to molecular forces. In long straight pipes, the velocity is near zero near the wall, and reaches its maximum in the center, along the axis of the pipe. The velocity distribution is parabolic, the maximum velocity (at the axis) is twice the average velocity. Since each laminum or layer of fluid retains its identity, this flow is called `laminar`. "
For a vertically oriented stack, (angle greater than 45.degree. to the horizon) and with laminar, low velocity flow, the gas velocity is practically zero along the inner surface of the stack. It is possible therefore for air to enter the top of the stack, along the edge of the stack, and flow down the inner surface of the stack in this narrow annular zone, of very low gas velocity. There will be consequent mixing of the downflowing air with the combustible gas, which can provide an explosive mixture. This presents a real danger to the operation of the flare stack.
It is to be noted that, when all else is equal, this air entry can occur only when the essentially static mass of gases contained within the vertically-oriented flare and flare-riser is composed of gases at molecular weight less than 28.97 (air), and therefore buoyant in respect to air. But even if the flare and flare riser are filled with heavy gases at the termination of the flare-relief period (where heavy gases present no air infiltration hazard) it is typical to make use of light gases which have molecular weights less than 28.97 for purge gases, and these gases soon replace the heavier gases contained within the flare and flare riser, and the air entry hazard becomes substantially constantly present because of this. A further source of air entry to the flare is due to wind-turbulence at the discharge point of the flare, and this effect is virtually constantly present at typical flare elevations, and air turbulence merits consideration for that reason because of wind movement at velocity is seldom less than 7feet/second and maybe as high as 135feet/second (5 mph to 90 mph).